Normally metal cans for various kinds of beverages and foods are manufactured as a three-piece metal can composed of a can body and top and bottom lids or a seamless metal can such as a two-piece can composed of a can body and a can lid.
Meanwhile, in order to prevent corrosion of the metal cans, a metal sheet coated by sticking a thermoplastic resin film to a surface of the metal sheet instead of conventional paint application is used for the metal cans. As this thermoplastic resin film used for the metal cans, a polyester film, particularly a copolymerized polyester film composed mainly of a polyethylene terephthalate-based resin, is used.
When these metal cans are used for the purposes of various kinds of beverages such as coffee, black tea, and tea and foods, normally retort sterilization treatment by steam at 110 to 140° C. is performed. There is a problem that water droplets adhere to the can lid of the three-piece metal can and the can bottom part of the seamless two-piece metal can at the time of this retort sterilization treatment and the film that was melted to become an amorphous state at the time of laminating is crystallized at these droplet-adhering parts and retort blushing (white spots) occurs. Therefore, various studies have been made.
As related arts to prevent the retort blushing (white spots), for example in PTL 1 (JP 1995-145252 A), a stretched film composed of a polyester composition that has a crystallization temperature of 65 to 130° C., a crystallization half time of 100 seconds or shorter, a second-order transition point of 40° C. or higher, and a melting point of 260° C. or lower and is obtained by mixing ethylene terephthalate and butylene terephthalate is described as a coating transparent film that is stuck to a metal can lid and is free from the occurrence of retort blushing (white spots).
In PTL 2 (JP 1995-330924 A), as a film excellent in the retort resistance, a metal-stuck stretched polyester film for forming processing is described that is a polyester film composed of 99 to 60 wt % of polyester having a melting point of 210 to 245° C. and a glass transition temperature of 60° C. or higher and 1 to 40 wt % of polyester that has a melting point of 180 to 223° C. and is composed mainly of polybutylene terephthalate, and has at most 300 ppm of free monomers.
In PTL 3 (JP 2003-268131 A), as a film that is excellent in design and is used to laminate a metal sheet and is free from whitening even when the film is subjected to heat treatment around the melting point or higher, the following polyester film is described. Specifically, the film is composed of 10 to 70 wt % of ethylene terephthalate and 90 to 30 wt % of crystalline polyester. The crystalline polyester is any of polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene-2,6-naphthalate (PEN). In the film, the half width of the recrystallization peak in a temperature decrease is equal to or smaller than 0.2.
In PTL 4 (JP 2008-143184 A), as a laminated metal sheet for a metal can body and a can lid material in which whitening of a film does not occur even when the film is subjected to crystallization treatment, a metal laminated sheet is described that is obtained by mixing 10 to 70 wt % of a polyethylene terephthalate-based resin and 90 to 30 wt % of a polybutylene terephthalate-based resin and has two or more melting point peaks.